In a railway vehicle such as a freight car, a car body is supported on the centerplate of a pair of spaced trucks. The coned wheels of the trucks travel a sinuous path along tangent or straight track as they continually seek a centered position under the steering influence of the wheel conicity. In traveling such a sinuous path, a truck will yaw cyclically with respect to the car body about the vertical axis defined by the vertical center line of the truck bolster centerplate.
Of course, the truck also yaws or rotates quasi-statically with respect to the car body in negotiating curved track. As a result of the aforementioned cyclic yawing, "hunting" can occur as the yawing becomes unstable due to lateral resonance that can develop between the car body and the truck. The reader is referred to prior U.S. Pat. No. 3,957,318 for further explanation of railway vehicle hunting phenomena, as well as disclosure of a vehicle side bearing which utilizes resilient bearing elements to provide restraining friction and shear stiffness at magnitudes which have been found successful for control of hunting. Such side bearings are in widespread use in modern railway rolling stock. The entire disclosure of prior U.S. Pat. No. 3,957,318 is incorporated herein and made a part hereof by reference.
Other prior side bearings commonly have comprised spring or elastomer biased steel caps retained by a base or cage. Disclosures of railway car side bearings known to applicant include the following: U.S. Pat. Nos. 2,301,372; 2,754,768; 3,518,948; 3,556,503; 3,628,464; 3,670,661; 3,796,167; 4,090,750; 4,080,016; and 4,567,833. Included among these are disclosures of resilient side bearing elements.
While the elastomer side bearing disclosed in Pat. No. 3,957,318 has proven generally successful in controlling high speed, empty car hunting for many freight cars, higher speed performance of laterally sensitive, longer freight cars and intermodal freight cars with extremely light empty car bodies requires more rigid side bearing longitudinal restraint and higher magnitude of car body to side bearing interface friction for hunting control. For this purpose, the instant invention contemplates a side bearing in which an elastomer column carries a metallic, higher friction generating cap with the cap being biased by the elastomer column. The elastomer column and the cap are so interlocked as to stiffen the longitudinal shear restraint of the side bearing, but at the same time retain the nearly linear elastic shear characteristic for longitudinal side bearing-to-car body motion which precludes impacting between metal parts as occurs in the case of prior art metal cap side bearings that utilize the vertical wall engagement between the side bearing top friction member or cap and the base member as a longitudinal shear restraint limit.
The instant invention insures that the linear shear restraint is sufficiently stiff to inhibit the onset or initiation of wheelset yaw and lateral movement. If the linear shear restraint is too soft, changes in wheelset direction are permitted to increase thus giving higher and higher wheel rail creep forces to be overcome. The lateral amplitude of the sinuous wheelset path becomes greater at a given speed sufficient to force the car body mass into lateral resonance on the truck, and the trucks begin to hunt. This condition is prone to occur with prior art all-elastomer bearing blocks when they are required to perform under severe dynamic conditions of frequent compression and shear cycles which cause increased operating temperatures. The higher temperatures in turn result in lower slope of the linear shear restraint and hunting is prone to occur at a lower threshold speed. Prior art side bearings with spring or elastomer biased rigid caps interacting with a rigid base usually have a soft or weak constraint between rigid limits, which permits the onset or initiation of wheelset yaw and lateral displacement with little restraint. The wheelset yaw momentum must then be suddenly impeded by the cap impacting longitudinally against the base. If these limits permit increased motion due to wear and impact, the side bearing permits truck hunting at lower and lower speeds thus creating a rapidly deteriorating condition and eventually causing the loosening of the bearing on the bolster and the wear plate on the car body. As the longitudinal free motion space between the cap and the base increases, the side bearing may become completely ineffective for hunting control.
Manufacturing tolerances for such prior art bearings require some lateral and longitudinal clearance between the cap and the base so that the vertical compression between vertical limits is unimpeded. As a result, all such prior side bearing designs are prone to acquire increased cap-to-base longitudinal clearance through wear from any combination of longitudinal impacting and vertical compression or extension.
With metal cap side bearings, lower bearing preloads are necessary so that the maximum swivel resistance to side bearing restraint for all dynamic side bearing load conditions is less than the rail-to-wheel sliding friction moment under the empty car.
Prior side bearings employing elastomeric means for biasing metal caps have been limited to short elastomeric biasing columns. Such short columns cause higher preload as well as greater side bearing friction level changes due to set up space variations which occur with dynamic car motions. Since the metal cap-to-car body wear plate coefficient of friction can exceed the rail-to-wheel coefficient, sufficient truck swivel restraining friction can be generated by the metal cap side bearing under the empty car body weight to require sliding or near sliding rail wheel conditions to turn the truck, causing accelerated wheel tread and wheel flange wear. Unduly high side bearing friction can also cause the wheel flanges to climb the rail and derail the truck.
Prior side bearings using helical spring loaded caps assembled on a metal base have inherently required a short spring with a high spring constant due to the vertical space limitation for assembly between the car body bolster structure and the truck bolster. Here again, extreme friction variations will occur from small side bearing vertical height changes.